In practice, when feeding screw-type extruders with rubber mixtures and/or thermoplastic masses, the operation of apparatuses of this kind relies on a high degree of continuity of the material composition and the feeding in order to achieve optimum results.
In most cases, the mixtures to be processed are delivered on pallets in the form of long strips, known as milled sheets, and are supplied to the screw-type extruder by conveyor belts, as is known for example from DE 23 64 539 A1.
In the case of the zig-zag or wig-wag mixtures in the form of milled sheets, the beginnings of the milled sheets are introduced directly into the feed unit or into the extruder funnel by hand or by means of specially developed auxiliary apparatuses. Subsequently, the mixture in the form of a milled sheet, optionally also two or more mixtures in the form of milled sheets, automatically continues on until the pallet is empty. A prerequisite in this case was also that the overall cross section of the mixture strip(s) matched, as far as possible, the optimum material reception by the extruder.
A feeding that is possibly too low as a result of a strip cross section that is too small results in a screw that is not sufficiently filled in this method, which can disadvantageously impact the plastification effect of the extruder and leads to significant underperformance. A milled sheet feeding cross section that is too large, however, results in obstruction of the funnel and disruptions to, or even interruptions of, the feeding.
An apparatus for automatically heating elastomer material in screw presses is known for example from DE 28 37 463 A1. Said apparatus comprises at least one intake roller for mixture material, which roller is arranged in front of the extruder screw with spacing therefrom and is driven independently of the screw. In this case, the material is supplied to the extruder by means of a funnel that is arranged in front of the extruder screw with spacing therefrom and is driven independently of the screw. Hydraulic motors are suitable as drives.
DD 2 32 876 A1 relates to an apparatus for continuously feeding an extruder by means of a draw-in roller and a draw-off roller that is upstream of the filling opening in the extruder. The combination of the draw-off roller and a counter-roller is intended to result in the draw-off and deceleration of the feed strip, where necessary.
In this case, in addition to the need for even feeding, it is also necessary to have an economical, largely automated operation. Depending on the intended use, there is also the desire to feed in two or more milled sheets at the same time, as a result of which a certain inevitable lack of homogeneity of the mixture disappears during the continuous shaping process.
In practice, the milled sheets are often fed simply by hand. Following the development of large screw-type extruders for cold feeding that have a high throughput capacity, this work has, however, reached its limits, and therefore the transition has been made to mechanical feeding.
DE 27 46 187 B1 discloses an apparatus for the controlled supply of a strip-shaped rubber mixture into the input opening in a screw machine comprising a funnel-shaped intake shaft that is arranged between a supply device and the input opening. A flap valve to which a constant tilting moment is applied covers the entire passage cross section of the intake shaft. A switch device for triggering switching pulses is provided in the end of the flap valve that projects rearwardly from the intake shaft in order to control the supply device by means of fixed proximity switches.
DE 22 44 059 C3 describes a supply apparatus for uniformly feeding, by weight, strip portions of a rubber mixture into a screw machine.
DE 19 61 632 A1 discloses an apparatus for supplying a rubber or plastics strip into the feed hopper, in which apparatus the conveyor and a cutting device are driven on the basis of the fill level of the feed hopper. In the process, the fill level of the feed hopper is read photoelectrically.
In another apparatus, known from DE 19 28 293 C3, screw presses are fed with plastically deformable materials by means of a controlled supply of a continuous strip, the supply of which being controlled by detecting the bead forming in the filling funnel of the screw press by means of a brake apparatus acting on the strip.
In an apparatus known from DE 77 10 535 U, the material supply is corrected as a result of a material jam in the feed opening to the extruder.
Furthermore, DE 35 05 128 A1 relates to an apparatus for producing films from a backing sheet made of thermoplastic material. A pair of draw-off rollers for the elastomer feed strip are rotated by an additional, controllable drive.